Image sensors are semiconductor devices that capture and process light into electronic signals for forming still images or video. Their use has become prevalent in a variety of consumer, industrial, and scientific applications, including digital cameras and camcorders, hand-held mobile devices, webcams, medical applications, automotive applications, games and toys, security and surveillance, pattern recognition, and automated inspection, among others. The technology used to manufacture image sensors has continued to advance at a rapid pace.
There are two main types of image sensors available today: Charge-Coupled Device (“CCD”) sensors and Complementary Metal Oxide Semiconductor (“CMOS”) sensors. In either type of image sensor, a light gathering photosite is formed on a semiconductor substrate and arranged in a two-dimensional pixel array. The photosites, generally referred to as picture elements or “pixels,” convert the incoming light into an electrical charge. The number, size, and spacing of the pixels determine the resolution of the images generated by the sensor.
Modern image sensors typically contain millions of pixels in the pixel array to provide high-resolution images. The electronic signals representing the image information captured in each pixel are transmitted to an Image Signal Processor (“ISP”) or other Digital Signal Processor (“DSP”) where they are converted into digital signals and processed to generate a digital image.
The quality of the digital images generated by an image sensor depends mostly on its sensitivity and a host of other factors, such as lens-related factors (flare, chromatic aberration), signal processing factors, system control-related factors (focusing and exposure error), time and motion factors, and other semiconductor-related factors (dark currents, blooming, and pixel defects.) In particular, pixel defects can easily deteriorate image quality if not accounted for. Too many pixel defects can affect image quality even if corrected.
Most pixel defects are introduced during the manufacturing of an image sensor wafer. As with any semiconductor manufacturing process, the manufacturing of an image sensor is not defect-free. The manufacturing process for producing image sensor wafers is similar to those used for devices such as flash memories and DRAM. However, because image sensors have a light-sensitive surface, defects that might not affect a purely electronic device can render an image sensor wafer useless.
Defects acquired during the manufacturing process of an image sensor wafer may be smaller or larger than a pixel. Some defects may cause a pixel or pixels to not function entirely. Others may only degrade pixel performance slightly or degrade it under dynamic operation or under stress conditions such as increased temperature. Other defects may affect a pixel and its neighbors and not be identifiable through a comparison of adjacent pixel responses. It may also be that illumination falling on a pixel does not produce the expected response. These defects may be caused by noise or fabrication errors, including dust particles, scratches, high leakage, circuit defects, color filter non-uniformity, microlens defects, and the like.
There are three main types of pixel defects: stuck high, stuck low, and abnormal sensitivity or blemish defects. A stuck high defect occur when the underlying pixel produces a high or near full scale output (i.e., white) regardless of the incident light intensity. Conversely, a stuck low defect occur when the underlying pixel produces a low or near zero output (i.e., black) regardless of the incident light intensity. Abnormal sensitivity or blemish defects occur when the underlying pixel produces an output that is different than a normal pixel when exposed to the same lighting conditions. These are harder to detect as the difference in the output value may be small.
Pixel defects may be detected with several techniques. The most common ones involve inspecting or testing image sensor wafers at the fabrication facility before the image sensor wafers are packaged into an image sensor device. These techniques make use of semiconductor testing equipment traditionally used to test semiconductor wafers or specifically designed to test image sensor wafers, including Automatic Test Equipment (“ATE”) systems provided by a variety of suppliers. Some of these ATE systems are capable of testing many image sensor wafers simultaneously.
Examples of ATE systems for testing image sensor wafers include the IP750EP test system provided by Teradyne, Inc., of North Reading, Mass., the T6171 system provided by Advantest America Corporation, of Santa Clara, Calif., the V93000 System-On-Chip (“SOC”) test system provided by Verigy, Inc., of Cupertino, Calif., and the Magnum iCP test system provided by Nextest Systems Corporation, of San Jose, Calif. The Magnum iCP test system, for example, may test up to forty image sensor wafers simultaneously.
Each image sensor wafer is tested under various illumination conditions to evaluate the image sensor wafer's response. A host processor is connected to each image sensor wafer by means of an interface, which includes a probe card mounted on a load board. The output of the image sensor wafers are sent to the host processor for analysis. The host processor typically contains a library of testing routines, including image processing routines, to evaluate the output of the image sensor wafers and detect defects.
Common tests include generating a series of bright and dark images at different resolutions to evaluate whether the Digital Number (“DN”) of each pixel in a given image sensor wafer corresponds to the correct amount of incident light. For example, a DN of 0 corresponds to a dark image (black) and a DN of 255 corresponds to a bright image (white). A wafer without any defects produces the same DN for each pixel when presented with a uniform light.
These and other currently-available ATE systems are typically intended for high-throughput, high-speed volume testing of image sensor wafers. As such, they may overlook subtle defects that cannot be detected until after assembly. The image processing routines in the host processor, for example, are not in general implemented with the goal of detecting small pixel defects such as blemish defects in the image sensor wafers being tested. Detecting subtle defects requires generating higher-resolution images, but at almost the same speed used for lower-resolution tests. These testers may not, for example, be able to identify subtle blemish defects that occur when uniform light incident on a given wafer fails to produce a uniform response across all pixels.
When that occurs, a defective image sensor wafer may be packaged into an image sensor device, thereby causing the device to be potentially rejected during testing by the device manufacturer or even later by a user of the device after its purchase. The cost incurred to the image sensor manufacturer, imaging device manufacturer, and eventually user, may be much larger than if the defect had been identified at an earlier stage of the manufacturing process, i.e., prior to packaging the image sensor wafer into the image sensor device.
Defects that are still present after an image sensor wafer has been packaged into an image sensor device can be detected with the use of software-based techniques, such as, for example, those described in U.S. Pat. Nos. 7,199,824 and 7,103,208. These techniques involve the use of an image processing unit coupled to the image sensor device to evaluate its response.
In U.S. Pat. No. 7,199,824, the output of each pixel and its surrounding, neighborhood pixels are examined. Pixels that differ by more than pre-determined thresholds from their surrounding pixels are determined to be defective. The defective pixels are then replaced with a value derived from their surrounding pixels.
In U.S. Pat. No. 7,103,208, an image of a predetermined scene is captured by the image sensor device and processed by a processor coupled to the device to identify defects. The defects are identified by a series of image processing functions, including applying an edge detector to enhance the edges in the captured image. The image sensor device is cleaned between two passes of the image processing functions to eliminate any defects detected due to dirt in the transmissive surface of the device.
An alternative provided in U.S. Pat. No. 7,209,168 integrates a pixel detection and correction mechanism in the image sensor package itself at the expense of higher packaging and processing costs. With device manufacturers pushing for lower costs and higher quality, there is a need to find as many defects and as early as possible in the assembly process of an image sensor device. Information about defects identified early in the assembly process can more quickly result in preventive measures.
Accordingly, it would be desirable to provide an apparatus and method for testing image sensor wafers that are capable of identifying pixel defects prior to packaging the image sensor wafers into image sensor devices. In particular, it would be desirable to provide an image sensor testing apparatus and method that can identify various pixel defects without sacrificing testing speed and throughput.